If B_TABLE.VALUE2 never contains 'X' and half of A_TABLE.VALUE1 are 'Y'
what would be the cost of executing this statement? A_TABLE.VALUE1 is
indexed and B_TABLE.VALUE2 does not have an index.

Would it be half of A_TABLE rows times all of B_TABLE rows? Or would
the Not Exists statement be evaluated for every row in A_TABLE making it
A_TABLE rows times B_TABLE rows?

SELECT *
FROM A_TABLE at
WHERE
at.VALUE1 = 'Y' AND
NOT EXISTS (
SELECT 1
FROM B_TABLE bt
WHERE bt.VALUE2 = 'X');

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